1. Field of the Invention
This invention relates to foam systems for extinguishing hazardous flammable liquid fires.
2. Description of the Prior Art
Conventionally, foam systems of the above-mentioned type have employed fixed displacement pumps for supplying foam liquid concentrate via supply conduits to one or more of the discharge outlets of a water pump. Where the system is truck-mounted, both pumps usually are driven by the truck motor via conventional power take off arrangements. The output pressures of both pumps are kept in balance, either automatically or manually. The concentrate supply conduits lead to pressure drop inducing devices which admit the concentrate into the water pump discharge outlets at flow rates governed by the flow rates of the water being pumped therethrough. Proportioning valves in the concentrate supply conduits operate either to selectively isolate the discharge outlets from the concentrate pump, or to control the amount of foam liquid concentrate being fed thereto.
The concentrate pump has adequate capacity to service all water pump discharge outlets under maximumn flow rate conditions, and it produces a constant output at a given motor RPM. This occurs irrespective of the number of water pump discharge outlets actually being fed with foam liquid concentrate.
This arrangement has certain decided drawbacks when, as is frequently the case, there occurs a simultaneous demand for both water and foam. When this situation is encountered, some of the water pump discharge outlets are fed with foam liquid concentrate in order to generate foam, whereas other water pump discharge outlets are kept isolated from the liquid concentrate pump, thereby enabling such outlets to provide the needed supply of water. The drive motor must necessarily be revved up to an RPM which supplies adequate power to the water pump so that it in turn can supply the needed water flow to all of the discharge outlets in use. The same RPM, however, causes the concentrate pump to develop excess output. Therefore, in order to maintain a balance between water pressure and foam liquid concentrate pressure, the excess output of the concentrate pump is recirculated back to the concentrate storage tank via a diaphragm operated pressure control valve.
The power which is consumed in developing the excess output of the concentrate pump is simply wasted. When liquid foam concentrate is being fed to only a small number of the water pump discharge outlets actually in use, the resulting power loss attributable to recirculation of liquid foam concentrate can be considerable, to the point where it can prevent the motor from driving the water pump at its rated maximum capacity. Moreover, as the foam liquid concentrate is being recirculated, its temperature is increased, and air is entrained. This can be detrimental to the more recently developed foam liquid concentrates, under some circumstances causing pre-foaming and degradation.